1. Field of the Invention
The present invention relates to microwave amplification tubes, such as traveling wave tubes or klystrons, and more particularly, to a coupled cavity microwave electron tube that produces an inverted slot mode and broadband response.
2. Description of Related Art
Microwave amplification tubes, such as a traveling wave tube (TWT) or klystron, are well known in the art. These devices are designed so that a radio frequency (RF) signal and an electron beam are made to interact in such a way as to amplify the power of the RF signal. A coupled cavity TWT typically includes a series of tuned cavities that are linked or coupled by irises (also called notches or slots) formed between the cavities. A microwave RF signal induced into the tube propagates through the tube, passing through each of the respective coupled cavities. A typical coupled cavity TWT may have thirty or more individual cavities that are coupled in this manner. Thus, the TWT appears as a folded waveguide and the meandering path that the RF signal takes as it passes through the coupled cavities of the tube reduces the effective speed of the signal so that the electron beam can effectively operate upon the signal. Thus, the reduced velocity waveform produced by a coupled cavity tube of this type is known as a xe2x80x9cslow wave.xe2x80x9d
Each of the cavities is further linked by an electron beam tunnel which extends the length of the tube and through which an electron beam is projected. The electron beam is guided by magnetic fields which are induced in the beam tunnel region and the folded waveguide guides the RF field periodically back and forth across the drifting electron beam. Thus, the electron beam interacts with the RF signal as it travels through the tube to produce the desired amplification by transferring energy from the electron beam to the RF wave.
Klystrons are similar to coupled cavity TWTs in that they can comprise a number of cavities through which an electron beam is projected. The klystron amplifies the modulation on the electron beam to produce a highly bunched beam containing a RF current. A klystron differs from a coupled cavity TWT in that the klystron cavities are not generally coupled. A portion of the klystron cavities may be coupled, however, so that more than one cavity can interact with the electron beam. This particular type of klystron is known as an extended interaction output klystron.
For a coupled cavity circuit, the bandwidth over which the amplification of the resulting RF output signal occurs can be controlled by altering the dimensions of the cavities and irises, and the power of the RF output signal can be controlled by altering the voltage and current characteristics of the electron beam. More specifically for the bandwidth, as the cavity narrows it propagates higher frequencies and as the iris narrows it propagates fewer frequencies.
There are generally two frequency bands of interest in which propagation can occur. The lower frequency band is referred to as the xe2x80x9ccavity passbandxe2x80x9d because its characteristics are controlled largely by the cavity resonance condition. The upper frequency band is referred to as the xe2x80x9ciris passbandxe2x80x9d and its characteristics are controlled mainly by the iris resonance condition. Normally, the cavity passband is used for interaction with the electron beam. As the length of the iris increases, the cavity resonance condition, usually appearing at the 2xcfx80 point on the lower passband of the dispersion curves, changes position with the iris resonance condition that appears at the 2xcfx80 point on the upper passband. When this passband mode inversion occurs (cavity passband and iris passband trading relative positionsxe2x80x94also known as inverted slot mode), it provides an advantage in preventing drive-induced oscillations and thus no special oscillation suppression techniques are required. Note that the mechanism of exciting the oscillations with a decelerating beam crossing a cavity resonance point is well known.
Unfortunately, to produce this passband mode inversion, the iris length is usually to such an extent that it wraps around the electron beam tunnel. This has the disadvantage of introducing transverse magnetic fields when the iris lies in an iron pole piece. Furthermore, a significant problem with RF amplification tubes is the efficient removal of heat. As the electron beam drifts through the tube cavities, heat energy resulting from stray electrons intercepting the tunnel walls must be removed from the tube to prevent reluctance changes in the magnetic material, thermal deformation of the cavity surfaces, or melting of the tunnel wall. The excessive iris length and corresponding reduction in the amount of metal results in a longer heat flow path around the iris. Thus the ability to remove heat is significantly reduced along with the overall coupled cavity circuit""s thermal ruggedness.
Accordingly, it would be desirable to provide a coupled cavity circuit having an iris that produces the passband mode inversion without the excessive iris length. Also, it would be desirable for the coupled cavity circuit to have a broadband frequency response while preventing drive-induced oscillations so that no special oscillation suppression techniques are required. Furthermore, it would be desirable for such a coupled cavity circuit to offer a significant increase in the amount of metal that is provided around the electron beam tunnel such that a passband mode inversion occurs without an increase in transverse magnetic fields or degradation in thermal ruggedness.
In accordance with the teachings of the present invention, a coupled cavity circuit is provided with an iris that produces passband mode inversion such that the iris mode passband is at a lower frequency than the cavity mode passband. In addition, the coupled cavity circuit also provides broadband frequency response while preventing drive-induced oscillations so that no lossy material is required within the coupled cavity circuit. Furthermore, the coupled cavity circuit provides these advantages without requiring an excessive iris length and thus avoids any severe increase in transverse magnetic fields or degradation in thermal ruggedness.
In an embodiment of the present invention, a microwave electron tube, such as a traveling wave tube or an extended interaction klystron, comprises an electron gun for emitting an electron beam through an electron beam tunnel to a collector that collects the electrons from the electron beam. A slow wave structure is disposed along the electron beam path and defines an electromagnetic path along which an electromagnetic signal interacts with the electron beam. The slow wave structure has at least one coupled cavity circuit comprising at least one iris disposed between a first cavity and a second cavity for coupling the electromagnetic signal between the first cavity and the second cavity. The iris is disposed between the electron beam tunnel and a sidewall of the tube with the iris being symmetrical about a perpendicular axis of the electron beam tunnel. The iris has a center portion with a first width and flared ends with a second width that is greater than the first width. The flared ends wrapping partially around the electron beam tunnel.
In a second embodiment of the present invention, the coupled cavity circuit of the slow wave structure has a rectangular shape. The iris has a rectangular central portion that extends substantially across one sidewall of the tube. The iris has flared ends that form a triangular region at each end of the central portion. The triangular regions have a hypotenuse that is adjacent to the electron beam tunnel and a side that extends part way along a sidewall of said tube that is adjacent to the one sidewall of the tube.
If there is more than one coupled cavity circuit, the irises can be in line, staggered, or on opposite sides of the tube. There can also be more than one iris per coupled cavity circuit with the irises in line or staggered from each other. The iris shape provides the inverted slot mode condition and broadband response without excessive iris length.
A more complete understanding of the coupled cavity circuit will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly.